The present invention relates to a sensing device and, more particularly, to a penetration sensor for use in physical security systems.
Conventional physical security systems designed for full coverage protection generally employ a breakwire system for detecting entry through the boundaries, such as walls, floors or ceilings, of a secured area. A breakwire system consists of a thin wire which is routed in a serpentine fashion over the boundary surfaces of the area to be protected. Current is then passed through the wire in a continuous manner so that any penetration of the boundary surface by an object will break the wire and interrupt the current flow. The interrupted current flow is detected by electronic circuitry which then sounds an alarm.
Although breakwire systems are in widespread use, there are several disadvantages associated with these systems. If it is necessary to protect the secured area from very small penetrations, such as small diameter drilling, smaller diameter wire must be employed and spaced closer together to provide for adequate sensing. This significantly increases the cost of the breakwire system. Additionally, since the breakwire system will only detect the first breakage of the wire, subsequent penetrations through the boundary surface will not be detected by the system.
Transducers employing materials having both piezoelectric and pyroelectric characteristics, such as poled polyvinylidene fluoride films, have also been used in physical security systems. For example, U.S. Pat. No. 4,706,069 discloses a physical security system with a plurality of such transducers mounted on the interior walls of a secured area. Each transducer comprises a single layer of polyvinylidene fluoride film with electrodes, an adhesive layer for mounting the transducer on the wall and a protective overcoat. These transducers are capable of detecting both temperature changes and vibrations within the wall. The signal produced by a stimulated transducer is supplied to a signal processor which, based on the generated waveform, recognizes the detected activity. Thus, if the signal corresponds to a single impact, such as a baseball or wind-blown object, an alarm signal would not be generated. However, if the generated waveform indicates a sudden increase in temperature, such as a fire or an attempted break-in using a torch, an alarm signal would be generated by the system.
The single layer transducer used in U.S. Pat. No. 4,706,069 would also produce an output signal if the ambient temperature of the secured area should increase or decrease, such as when the heating or cooling system is activated. The signal processing equipment coupled to the transducer is capable of recognizing the waveform which is produced when smaller temperature changes are detected, and an alarm signal is not generated by the system. However, instead of relying on signal processing equipment, the transducer can be designed to provide for temperature compensation. An example of such a transducer is disclosed by A. L. Taylor in U.S. Pat. No. 3,971,250 issued July 27, 1976. This transducer comprises two pyroelectric sensing elements which are sandwiched around a layer of thermally and electrically insulating material. If both of the sensing elements are equally varied in temperature, such as when the ambient room temperature changes, the outputs from the individual sensing units cancel and there is no overall signal produced by the sensor. However, if the temperature of only one of the sensing units is varied, such as when infrared radiation impinges on one of the sensing elements, an overall output signal is produced by the sensing device.
Although the sensor and associated electronics disclosed by Taylor provide for adequate temperature compensation, the Taylor patent fails to disclose a sensor or system for interrogating the operational state of each of the individual sensing elements. Since the piezoelectric and pyroelectric activities of the materials used in the sensing medium in Taylor can degrade over time, a need has developed for a sensor and sensing system which allow for the operational state of the sensing units to be tested to ensure that the sensing mediums retain their piezoelectric and pyroelectric activities.
The present invention provides for such an interrogatable sensor and an interrogation system for testing the operational state of the sensing units.